1. Field of the Invention
The present invention relates to a high voltage generation circuit, and particularly relates to a high voltage generation circuit responsive to a clock signal to generate a desired voltage.
2. Description of the Background Art
The Dynamic Random Access Memory (DRAM), the Static Random Access Memory (SRAM) and the like are famous as semiconductor memory devices. They are both volatile memories and data stored therein vanishes if the supply voltage is not applied.
In contrast, the flash memory can retain storage data even if the supply voltage is not applied. The flash memory stores charge in a floating gate within a memory cell to achieve nonvolatile storage. In the flash memory, the state of storage of 1 or 0 is generated by injecting/drawing the charge into/out of the floating gate which retains the charge. In this case, the Fowler-Nordheim (FN) tunneling phenomenon or the channel hot electron is used to inject/draw the charge into/out of the floating gate. In order to generate such a state, voltage higher than the operating supply voltage of a device is generally required.
Referring to FIG. 14, a configuration of a high voltage generation circuit which generates such high supply voltage is described.
FIG. 14 is a circuit diagram showing one example of a specific configuration of a conventional high voltage generation circuit 2000.
High voltage generation circuit 2000 illustrated in FIG. 14 includes a plurality of capacitor elements 221.1, . . . , 221.n, a plurality of rectifying elements 220.1, . . . , 220.n+1 and a ring oscillator 222.
The plurality of rectifying elements 220.1, . . . , 220.n+1 are connected in series between an input terminal 230 receiving high voltage (Vcc) and an output terminal 232. The plurality of capacitor elements 221.1, . . . , 221.n each has one terminal connected to a respective one of output nodes of rectifying elements 220.1, . . . , 220.n. Ring oscillator 222 generates two periodic pulses .phi. and /.phi..
The timing charts of FIG. 15A and FIG. 15B respectively show periodic pulse .phi. and periodic pulse /.phi.. As shown in FIGS. 15A and 15B, the phase of periodic pulse .phi. is an inverted one of the phase of periodic pulse /.phi..
The other terminals of capacitor elements 221.1, 221.3, . . . respectively receive periodic pulses .phi. from ring oscillator 222. The other terminals of capacitor elements 221.2, 222.4, . . . respectively receive periodic pulses /.phi. from ring oscillator 222.
An operation of the conventional high voltage generation circuit 2000 is next described.
If one end of capacitor element 221.i (1.ltoreq.i.ltoreq.n) receives a periodic pulse at an H level, current flows in the direction of rectifying element 220.i+1. If capacitor element 221.i receives a periodic pulse at an L level, current flows from the direction of rectifying element 220.i. As a result, a high voltage Vout is generated at output terminal 232 placed at the last stage.
Specific examples of rectifying elements 220.1, . . . , 220.n+1 are the PN diode, the MOS transistor and the like.
If the PN diode is used as the rectifying element, for example, the threshold voltage is 0.7V-0.8V. If the MOS transistor is used as the rectifying element, the element also has a threshold voltage of an almost equivalent value.
Therefore, in the conventional high voltage generation circuit 2000, voltage larger than the threshold voltage must be applied to both ends of each rectifying element in order to allow current to sufficiently flow and generate high voltage. If the supply voltage (Vcc) is reduced, the amplitude of voltage of two periodic pulses .phi. and /.phi. output from ring oscillator 222 decreases and current does not sufficiently flow.
In a semiconductor memory device represented by the flash memory, the supply voltage tends to be decreased. Therefore, if the conventional high voltage generation circuit 2000 is applied to the flash memory as it is, generation of high voltage required for an internal operation becomes difficult as the supply voltage is decreased.